1. Scope of the Invention
The present invention pertains to an apparatus for controlling the discharge of granular or pelletized materials from a storage tank and, more specifically, to a control device of this type especially designed to ensure the continuous and uniform discharge of pelletized materials from a cooler for pellets.
2. Prior Art
The controlled discharge of granular or pelletized materials from a storage tank is an urgent problem in practically all storage systems currently known, and it is of particular importance with respect to pelletized materials.
Processes for the treatment or beneficiation of powdered and/or particulate materials by compaction and extrusion, commonly called pelletization, are widely known and utilized in the state of the art to facilitate handling and to reduce the storage, packaging, and transport costs of these materials, in addition to providing for their greater durability.
In a typical pelletizing machine, the powdered and/or fine-particulate material to be pelletized is fed continuously into a compaction chamber with an extrusion die at one end.
A feed arrangement in the compaction chamber distributes the material uniformly on steel rollers working together with the internal surface of the die, the conjoined rotation of which strongly compresses the material and thus forces it through the die orifices.
A cutting device, containing a plurality of knife blades, is located adjacent to the outer surface of the die at a distance that can be adjusted for cutting the material thus compacted into the desired lengths immediately after its extrusion so as to form pellets of the desired dimensions.
The compaction, compression, and extrusion of the material obviously cause a substantial increase in temperature; therefore, the pellets must be cooled in some way.
Various pellet coolers based on various cooling systems have been tested, and experience has revealed that the best results are obtained with coolers of the counterflow type.
In these coolers, the pelletized material to be cooled is fed in the descending direction into a heat-exchange chamber, while a stream of cooling air is blown in the opposite direction, i.e., upward.
Although these counterflow coolers provide satisfactory cooling of the pellets, they still present some problems, primarily with respect to the discharge or escape of the cooled material.
In fact, certain parameters and characteristics must be satisfied by such coolers. The following requirements can be mentioned: the uniform cooling of all the material; the facility for emptying completely for cleaning; the continuous and uniform discharge of the pellets; and, basically, a discharge mechanism that does not compromise the integrity of the pelletized material.
The problem of uniform cooling is readily solved by providing a cooling chamber with a circular cross section and with means that ensure the adequate distribution of the ascending stream of cooling air.
The other problems are not so easy to solve, however, and none of the commercially available pellet coolers is capable of satisfactorily solving the three problems simultaneously.
A pellet cooler available from Machinefabriek A. Wijnveen B. V. is supplied with a discharge device consisting of a pair of stationary gratings equipped with annular channels and a movable grate with a number of interconnected rings which move eccentrically to open and close the apertures of the stationary grates.
This discharge arrangement, due to the eccentric movement of the rings, does not provide for the uniform discharge of the pellets, nor does it protect their integrity, since they can be crushed between the apertures and the rings.
Another pellet cooling design, U.S. Pat. No. 4,683,665, available from Geelen Haelen Holland B. V., is supplied with a discharge device consisting of a pair of stationary grates equipped with nonaligned openings and a mobile grate moving back and forth between them to open and close the openings and thus to discharge the pellets.
However, this discharge design does not provide for the uniform and constant discharge of pellets, and it is also difficult to clean, since a certain quantity of pellets remains caught in the mobile grate at the extreme portions of its alternating movement.